Boot upper



FIG. 1 is a side elevational view of a boot upper according to my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is an opposite side elevational view thereof;

FIG. 5 is a top plan view thereof; and,

FIG. 6 is a bottom plan view thereof.

The dashed lines throughout the views form no part of the claimed design. 

The ornamental design for a boot upper, as shown and described. 